Method of activating electron emissive electrodes

ABSTRACT

A method of treating metal oxide semiconductor electrodes of the type utilized in fluorescent lamps to increase their electron emissivity and to lower their ignition voltage. The electrodes are heated in the absence of air to render them oxygen deficient and then cooled before exposure to air.

United States atent 1191 [11] 3,842,469 Menelly 1451 Oct. 22, 1974 [54] METHOD OF ACTIVATING ELECTRON 2,142,331 1/1939 Prescott, Jr 29/2517 x EMISSIVE ELECTRODES 2,275,886 3/1942 Bondley et a1 29/25.l4 X 3,492,142 12/1949 Germeshausen 313/346 DC Inventor: Richard y, Danvers, s- 3,560,790 2/1971 Vollmer et a1 313/311 Assigneez International Telephone and 3,704,927 12/1972 Toomey et a1 316/12 X Telegraph Corporation, Nutley, NJ. [22] Filed: Aug. 20, 1973 Primary Examiner-Roy Lake Assistant Examiner-James W. Davie [21] Appl' Attorney, Agent, or Firm-John T. OHalloran;

Related US. Application Data Menotti J. Lombardi, Jr.; Peter C. Van Der Sluys [63] Continuation-in-part of Ser. No. 200,398, Nov. 19, 1971, abandoned, which is a continuation-in-part of Ser. No. 150,310, June 7, 1971, abandoned.

[57] ABSTRACT [52] US. Cl 29/25.]1, 313/346 R, 316/1 [51] llnt. Cl. H01 j 9/00 A method of treating metal oxide semiconductor elec- Field Search 2 /2 11, -14, 2 -17; trodes of the type utilized in fluorescent lamps to in- 316/12; 313/346 R, 346 DC, 311, 109, 213, crease their electron emissivity and to lower their igni- 217 tion voltage. The electrodes are heated in the absence of air to render them oxygen deficient and then cooled [56] References Cited before exposure to air.

UNITED STATES PATENTS 1,849,594 3/1932 Schrotkr 29/25.l7 X 7 Claims, 1 Drawing Figure 7 I l I I! II II III! 1 VACUUM- 7E EXHAUST PAIENKOWZZIBH 3.842.469

WINNIEWHWM $1! II IIIIIIIIIIIIIIIIIIIIIII II [III/III III. "I'm! 7 VACUUIVP/IE sou/ace EXHAUST OF //4 INER r aAs INVENTOR RICHARD A. MENLL Y ATTORNEY METHOD OF ACTIVATING ELECTRON EMISSIVE ELECTRODES CROSS-REFERENCE TO RELATED APPLICATIONS This is a continuation-in-part of my copending appli cation, Ser. No. 200,398, entitled Method of Activating An Electron Emissive Electrode, filed Nov. 19, 1971, now abandoned, which is a continuation-in-part application of application Ser. No. 150,310, entitled A Method of Activating Electrodes, filed June 7, 1971 now abandoned.

BACKGROUND OF THE INVENTION This invention relates to a method of activating or increasing the emissivity of metal oxide semiconductor electrodes used in fluorescent lamps and, more particularly, to such a method which is performed outside the fluorescent lamp in which the electrode is to be used.

Electrodes are utilized in fluorescent lamps to provide free electrons, thereby enabling current flow in the florescent tube and may therefore be called cathodes. These cathodes are normally one of two types, the first of which is called a hot or thermionic cathode, and the other of which is usually referred to as a cold cathode.

A hot cathode of the type well known in the art is normally made by painting, dipping or otherwise adhering to a co-precipitate of triple carbonates, usually strontium carbonate, calcium carbonate and barium carbonate to a coil of tungsten wire. The coil or filament of tungsten wire is utilized to heat the material adhering thereto, this heat causing electrons to be emitted from the material. However, it is well known that the alkaline earth carbonates, mentioned above, are poor emitters of electrons and therefore, to increase the emissivity of the material on the filament, so as to reduce the voltages required for ignition of the completed fluorescent lamp, the alkaline earth carbonates on the filament are subjected to an activation process. This activation process usually comprises the following steps: the cathode, prior to activation, is sealed into the fluorescent lamp in which it is to be used, the lamp envelope is evacuated to a pressure of approximately 50 microns, and then is continuously flushed with an insert gas, such as argon. While the electrode is in this atmosphere of argon at a pressure of 50 microns, a low voltage, for example 2 volts, is applied to the high resistance filament so as to heat the carbonates adhering thereto. In sequential steps the voltage on the filament is increased from the aforementioned 2 volts to approximately 10 volts thereby raising the tempera ture of the material adhering to the filament to l,100 to l,200C, this temperature reducing the material on the filament to the oxides of the alkaline earth metals. It may here be noted that the reason the voltage across the filament is raised in steps is due to the fact that the coefficients of expansion of the tungsten filament and of the triple carbonates are widely divergent and a sudden raising of the temperature of the filament and the carbonates would result in pieces of the adherent material falling off the filament which is, of course, undesirable since this results in a loss of emissive material which, in turn, results in a shorter lamp life. Thus, after this activation heating there is provided in the fluorescent lamp a tungsten filament coated with an alkaline earth oxide including the one here of interest, barium oxide. Further, it is presently believed in the art that a monolayer of barium metal exists on the surface of the oxide coating. It is appropriate here to note that the reason the cathode is activated within an inert gas atmosphere, for example argon, is that the barium oxide, noted above as the alkaline earth oxide of interest which results from the activation step, readily adsorbs contaminants such as oxygen, carbon dioxide, carbon monixide and water vapor from air, reducing the emissivity of the cathode and resulting in an increased ignition voltage which obviously has a deleterious effect on the operation of the lamp. The foregoing activation procedure is accomplished in approximately six minutes, or about half the time presently required in the art for the processing of a fluorescent tube from a phosphor-coated glass cylinder into a completed lamp.

Cold cathodes well known in the art normally comprise iron or nickel plated iron cups into which is placed the above-discussed co'precipitated triple carbonates. Here too the activation required to increase the electron emissivity of these electrodes or cathodes and thereby reduce the lamp ignition voltage requires their heating to break down the carbonates into oxides. However, since these cold cathodes do not contain a heating filament as the thermionic or hot electrodes do, the usual method of activation includes sealing the cup cathodes into the fluorescent lamp in which'they are to be utilized, reducing the pressure of the air inside the lamp to approximately one half atmosphere, and applying a very high voltage, in the order of 5,000 to 10,000 volts across the lamp electrodes. This ionizes the air in side the lamp and the cup electrodes are thus subjected to high speed ionic bombardment which heats the electrodes as did the filament in the above-mentioned thermionic cathode. The high voltage is maintained for a period of approximately five minutes during which time the pressure in the lamp is continuously reduced thereby increasing the speed of the ions impinging upon the electrodes, since the reduction in pressure increases the mean free path the ions may travel. Here too, as was the case in the above-mentioned thermionic cathode, the material in the cup reaches approximately 1,1001,200C causing a breakdown of the carbonates into oxides of the alkaline earth metals, thus resulting in an electrode or cathode which has a high degree of electron emissivity.

Again, these cold cathodes must be activated within the fluorescent lamp tube in which they are to be used. As previously stated, barium oxide cannot be allowed to contact air since it will adsorb the aforementioned contaminants therefrom, rendering the electrodes less electron emissive and causing an increase in lamp ignition voltages.

Metal oxide semiconductor electrodes of the type formed by reacting alkaline earth peroxides with refractory metal powders are finding increasing usage within the lamp industry. Fluorescent lamp life is related to the amount of emissive material that can be incorporated into the lamp electrode. The alkaline earthrefractory metal electrode is formed by a process which results in a fusion between these materials and a supporting electrode wire. Large quantities of the material can therefore be formed into the electrode resulting in longer life than standard emissive coated electrodes.

Metal oxide semiconductor electrodes as currently employed in the lamp industry have the disadvantage of activation by oxygen removal within the lamp envelope. This is currently performed by sealing the electrodes within the lamp envelope and subsequently heating the electrodes by some mechanism such as radio frequency heating or electron and ion bombardment by generating a plasma between the electrodes within the lamp. This method is time consuming and inefficient since the walls of the discharge lamp itself can become a source of oxygen to the electrodes and the entire lamp electrodes assembly must be simultaneously heated and evacuated to insure the removal of oxygen.

Activation of the electrodes within the lamps therefore takes several minutes and the possibility of back contamination of oxygen to the electrodes is quite high. Production speeds are limited due to the long activation times involved so that lamps processed employing these electrodes are relatively expensive to manufacture.

It will be clear that the aforementioned methods of activating the electrodes utilized in fluorescent lamps are unsatisfactory in a number of respects. Firstly, each electrode must be activated within the lamp cylinder with which it is to be utilized since it is necessary that after activation the electrode be protected from the air, which is a contaminating atmosphere.

Secondly, since the length of time required to activate the electrodes, approximately 5 to 6 minutes, depending on the type of electrode utilized, is approximately half the time required to convert a phosphorcoated glass envelope into a completed fluorescent lamp, it is seen that the electrode activation step is the major factor in limiting lamp manufacturing speeds.

Thirdly, if either of the aforementioned types of cathode is defective, this fact is not known until the electrode has already been sealed into the lamp tube and thus requires the discarding of an otherwise satisfactory phosphor-coated glass cylinder.

Finally, although the purpose of the activation process is to reduce the voltage required for firing a fluorescent lamp, the above-discussed methods of activating electrodes result in firing or ignition voltages which are higher than desirable. Particularly in the case of cold cathodes, the high firing voltages required are a major factor in preventing their widespread use.

SUMMARY OF THE INVENTION The present invention contemplates a method of activating a metal oxide semiconductor electrode. These electrodes are not readily electron emissive after the fusion process takes place since the valencies of the constituent elements are presumably filled. Heating these electrodes in the absence of air causes oxygen to be transported out of the material thereby resulting in an oxygen deficiency which is necessary in degenerate semiconductors to render them electron emissive. The oxygen deficient electrodes are then cooled to room temperature prior to exposing the electrodes to air.

Metal oxide semiconductor electrodes activated in this manner emit electrons with relative ease. The explanation for the ease with which the electrodes are emitted from these compounds after oxygen deficiency has beenestablished is not fully understood but is believed to be related to the ease in ionic conductivity which occurs when possible existing valencies are unfilled.

The main object of this invention is to provide a method for activatingmetal oxide semiconductor type electrodes outside of the discharge tubes in which they are utilized.

5 Another object of this invention is to provide a speedy economical method for treating electrodes resulting in a substantial reduction in lamp process costs.

It is a further object of this invention to provide a 0 method for activating electrodes which results in lower ignition voltages.

A further object of this invention is to provide a method of rendering metal oxide semiconductor electrodes thermionically emissive for electrons by creating 15 an oxygen deficiency within the structure of the semiconductor compound.

Another object of this invention is to provide a method of removing oxygen from metal oxide semiconductor type electrodes without back-contaminating the electrodes with oxygen.

It is a feature of this invention that the instant method may be utilized to activate large numbers of electrodes at the same time.

It is yet another feature of this invention that electrodes activated according to the instant method are available for testing prior to their installation into the fluorescent tubes with which they are to be utilized.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 describes the apparatus used in activating electrodes according to the inventive method.

DESCRIPTION OF THE PREFERRED EMBODIMENT While the instant activation method may be utilized with a number of different electrode structures of the type which must be activated in order to increase their electron emissive properties, for example, the electrodes utilized in neon tubes, electron tubes and fluorescent lamps, the method will be discussed primarily with regard to fluorescent lamp electrodes and more specifically with regard to the fused cup electrode described in copending application, Ser. No. 149,971, entitled Fused Cup Electrode Having A Bulk Density Gradient Structure,. filed on June 4, 1971 by Richard A. Menelly and assigned to the assignee of the present invention.

Electrode l, which was manufactured according to the teachings of the aforementioned copending application, by an exothermic reaction between tantalum powder and barium peroxide, said reaction taking place within iron cup 2, is placed in a boat or tray 3, which may, for example, be made of tantalum. It should be noted here that in the normal activation process a large number of electrodes will be placed within tray or boat 3, but for purposes of illustration, only'one electrode is shown. A blanket 4 of a porous, non-melting, non-reactive material, for example, Grade 970-A Fibre Frax type refractory paper or asbestors or quartz wool, surrounds electrode 1 within tray 3. A cover 5, which may be made of the same material as tray 3, in this example tantalum, and having a plurality of small diameter holes 6 formed therein, covers tray 3.

The covered tray, with electrode inside, is placed within' an enclosure 7 which may be, for example, a standard bell jar. The covered tray is connected via cover terminals 8 and 9, and switch 10, to a source of electrical energy 11. An environment which is substantially oxygen free with respect to electrode 1 is now provided for said electrode within enclosure 7. This environment is preferably a high vacuum, in the order of l" tort, or, alternatively, may be an inert gas atmosphere, comprising, for example, argon, xenon or krypton gas. Since the environment may be provided in various ways, the apparatus utilized to provide two such environments has been illustrated. A standard vacuum exhaust 12 coupled to enclosure 7 via valve 13 may be utilized to provide the above-mentioned vacuum environment, and a source of inert gas 14 coupled via valve 15 to enclosure 7 may be utilized to provide the inert gas atmosphere. It will of course be clear that any environment existing within enclosure 7 also exists within covered tray 3, due to the free flow of gas between covered tray 3 and enclosure 7 through holes 6 in cover 5, which holes have been provided for this purpose. After the desired environment is provided within enclosure 7, switch 10 is closed. As current begins to flow through the tantalum tray 3 and tantalum cover 5, the electrical energy applied is so selected relative to the resistance 'of the tantalum tray and cover that the covered tray and the electrode 1 located therein are heated to a temperature of approximately l,l75C 25C. It is appropriate to note at this time that although a source of electrical energy has been here illustrated as providing the means for heating electrode 1 to the aforementioned desired temperature, it will be clear that any source of energy may be utilized to provide the required heating and, for example, a standard muffle furnace may be used or, alternatively, a high frequency coil of the type well known in the art may be used. If one of these latter methods of heating electrode 1 is utilized, it is not necessary that tray 3 and cover be of a conductive material such as tantalum, but another material, such as a high melting temperature ceramic may be substituted therefor.

Returning now to the example, electrode l is maintained at the temperature of 1,175C 25C for a period of approximately. 1 0 minutes, at which time switch is opened and the tantalum tray and cover, and the electrode located therein, are allowed to cool to room temperature; i.e., between 10C and 50C. At this time the activation process is complete and the electrode is in its activated condition. The activating temperature is not critical and need only be sufficiently high as to create an oxygen deficiency in the material utilized in filling cup 2 and to outgas cup 2. For example, the electrode may be activated by maintaining it at a temperature of 900C for a period of up to one hour, while at l,600C the electrode will be activated within nine minutes. One method for activating the electrode of the instant invention comprises heating the electrode in an oxygen-free environment to a temperature exceeding 525C and less than the vaporization temperature of the electrode constituents until the electrode is rendered oxygen deficient. A preferred embodiment of the instant invention consists of heating the electrodes within an oxygenfree environment; for example, within a vacuum chamber evacuated to a pressure of at least 1 militorr to at least 1,000C for at least 1 minute to render the electrode oxygen deficient. It will be clear, however, that the temperature utilizedshould not exceed that temperature at which the elements comprising the electrode begin to evaporate.

It will be noted that electrode 1 has been covered with a blanket of porous, non-reactive material 4, such as 'Fibre Frax paper. This is not necessary for the activation of electrode 1 and is included merely to protect tantalum tray 3 and tantalum cover 5. Thishas been found advisable, since during the activation heating of electrode 1, particles of barium and barium compounds sputter off, resulting in a chemical reaction between the barium and the tantalum which, in turn, has resulted in the destruction of the tantalum container.

As stated above, after the electrode has been cooled to room temperature in the oxygen free environment,

I it is in its activated condition and may now be removed from the enclosure 7.

It has been further discovered that electrodes, such as electrode 1, made in accordance with the teachings of the aforementioned copending patent application, and activated within sealed fluorescent lamps by the usual method of ionic bombardment, have an ignition voltage of approximately 500 volts, whereas the same electrodes activated in accordance with the teachings of the subject invention have an ignition voltage of only 500 volts, a reduction of fully 20 percent.

It will thus be clear that many advantages are obtained by utilizing the subject method of activation. For example, a large number of electrodes may be activated at the same time and in an enclosure other than the fluorescent lamp in which they are to be utilized. Thus, substantial savings in lamp manufacturing time are realizable since each lamp need not be delayed during the manufacturing process for activation of its electrodes. Further, since the activated electrodes here produced by the subject process are reasonably air stable, they are available for testing prior to their insertion into the lamp envelopes and therefore any nonsatisfactory electrodes may be discarded prior to their sealing into the lamps with which they are to be associated. Finally, and of major importance, is the fact that electrodes manufactured in accordance with the subject process have a much lower ignition voltage than electrodes activated in' accordance with the teachings of the prior art. This allows either greater numbers of lamps to be connected in series to the same ballast circuit or, alternatively, allows smaller and less expensive ballast circuits to be used with a fixed number of lamps.

While the principles of the invention have been described in connection with specific structure, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of the invention as set forth in the objects thereof and in the accompanying claims.

I claim:

1. A method of providing an electrode for an electron emissive device wherein the electrode has improved electron emission characteristics, comprising the steps of:

selecting an electrode formed by an exothermic reaction between a refractory metal powder and an alkaline earth peroxide; heating said electrode in an oxygen free environment to a temperature exceeding 525C and less than the vaporization temperature of the electrode constituents until the electrode is rendered oxygen deficient; cooling said electrode in the oxygen free environment to room temperature; and exposing the electrode to atmosphere. 2. A method according to claim 1, wherein the oxygen free environment is a partial vacuum.

3. A method according to claim 2, additionally comprising the step of subjecting the cooled electrode to an inert gas prior to exposing the electrode to atmosphere.

4. A method according to claim 1, wherein the oxygen free environment is an inert gas.

5. A method according to claim 1, wherein a plurality of electrodes are activated simultaneously.

6. A method of treating electrodes of the type formed by a thermal reaction between refractory metal powders and alkaline earth peroxides comprising the steps of:

placing said electrodes in a suitable chamber;

evacuating said chamber to a pressure of at least 1 militorr vacuum;

heating said electrodes within said chamber to at least 1,000C for at least one minute to render said electrodes oxygen deficient; cooling said electrodes within said chamber to ambient room temperature; and

removing said electrodes from the chamber.

7. A method according to claim 6, additionally comprising the step of filling said chamber to at least one atmosphere pressure of an inert gas prior to removing the electrode from the chamber. 

1. A METHOD OF PROVIDING AN ELECTRODE FOR AN ELECTRON EMISSIVE DEVICE WHEREIN THE ELECTRODE HAS IMPROVED ELECTRON EMISSION CHARACTERISTICS, COMPRISING THE STEPS OF: SELECTING AN ELECTRODE FORMED BY AN EXOTHERMIC REACTION BETWEEN A REFRACTORY METAL POWDER AND AN ALKALINE EARTH PEROXIDE; HEATING SAID ELECTRODE IN AN OXYGEN FREE ENVIRONMENT TO A TEMPERATURE EXCEEDING 525*C AND LESS THAN THE VAPORIZATION TEMPERATURE OF THE ELECTRODE CONSTITUENTS UNTIL THE ELECTRODE IS RENDERED OXYGEN DEFICIENT; COOLING SAID ELECTRODE IN THE OXYGEN FREE ENVIRONMENT TO ROOM TEMPERATURE; AND EXPOSING THE ELECTRODE TO ATMOSPHERE.
 2. A method according to claim 1, wherein the oxygen free environment is a partial vacuum.
 3. A method according to claim 2, additionally comprising the step of subjecting the cooled electrode to an inert gas prior to exposing the electrode to atmosphere.
 4. A method according to claim 1, wherein the oxygen free environment is an inert gas.
 5. A method according to claim 1, wherein a plurality of electrodes are activated simultaneously.
 6. A method of treating electrodes of the type formed by a thermal reaction between refractory metal powders and alkaline earth peroxides comprising the steps of: placing said electrodes in a suitable chamber; evacuating said chamber to a pressure of at least 1 militorr vacuum; heating said electrodes within said chamber to at least 1,000*C for at least one minute to render said electrodes oxygen deficient; cooling said electrodes within said chamber to ambient room temperature; and removing said electrodes from the chamber.
 7. A method according to claim 6, additionally comprising the step of filling said chamber to at least one atmosphere pressure of an inert gas prior to removing the electrode from the chamber. 